When other fishing boats operated near a ship concerned exist, a transmission signal is transmitted from a fish finder equipped on another boat, and a comparatively strong reflection wave reflected to the water bottom is received by a fish finder equipped on the ship concerned and, thus the reflection wave appears as an interference wave in a detection image. JPA 2003-322678 discloses a technique for removing the interference wave. The interference removal technique compares reception data received based on a current transmission with reception data at the same depth among the reception data received based on a previous transmission, and sequentially sets, in the depth direction, a lower level data with as current reception data to be displayed at the depth concerned.
In JPA 2003-322678, an echo signal from the same target object, such as a school of fish, is received every time as a signal of substantially the same level. For this reason, when comparing the current reception data with the previous reception data at the same depth, and selecting the lower level data, the echo signal from the target object is seldom spoiled because both the reception data have substantially the same level. Meanwhile, the interference signal is often received as a signal of a stronger level than the echo signal from a normal school of fish or the like, at a different depth position for the previous time and the current time. In addition, the interference signals are less often received consecutively at the same depth. Therefore, even if the current reception data is compared with the previous reception data at the same depth and the lower level data is selected, data which is not the interference signal will be selected and, thus, the interference signal will not be displayed.
Meanwhile, in recent years, a fish finder provided with a high distance resolution where an echo from a small target object, such as a single fish, can be identified has been developed. However, if the above-mentioned interference removal function is applied to the fish finder with the high distance resolution, a problem may arise in which the echo from the single fish and the small target object will be spoiled as well as the interference wave. That is, it may be considered that, by the distance resolution in the depth direction being high, the single fish moves to a different depth during a transmission cycle, and relevance will be lost between the reception data at the same depth which are obtained by the receptions of the previous time and the current time (i.e., in the temporal direction). More specifically, by the single fish moving in the depth direction, if the previous reception data is at a predetermined level and the current reception data is substantially at zero, at a certain depth, the current reception data is selected for the depth concerned. Conversely, if the previous reception data is substantially at zero and the current reception data is at the predetermined level, the previous reception data will be selected. Therefore, the zero level will be selected for both the cases.